TORONTO - Some of the crude-oil tanker cars on the Canadian National Railway train that derailed and caught fire in New Brunswick on Tuesday appear to be of an older type that regulators have warned is vulnerable to leaks and explosions.

Of the five crude tank cars that derailed, two were older DOT-111 models, a CN Rail spokesman said on Thursday, citing information the company received from the Association of American Railroads.

The train derailed in a rural area near a small village in the eastern Canadian province of New Brunswick on Tuesday night. A total of 19 cars and one locomotive on the 122-car, four-locomotive train went off the rails.

Three cars were still burning on Thursday and one of them was a crude tanker.

The U.S. National Transportation Safety Board says older models of the DOT-111 are vulnerable to leaks and explosions. The cars have become a focal point in a debate on rail safety regulation as crude-by-rail shipments across the continent surge.

CN Rail did not specify whether the crude tanker still on fire was an old or new model or whether the newer versions of the DOT-111 fared any better than the older ones in the crash.

Three of the five derailed crude tank cars were new DOT-111 models that comply with higher U.S. standards ordered after October 2011, CN spokesman Mark Hallman said in an email.

"Two of the tank cars of crude oil that derailed in the New Brunswick incident are the older DOT-111 tank cars that CN and the rail industry are recommending be phased out or retrofitted," Hallman said, adding that the vast majority of tank cars are owned by shippers and by rolling-stock leasing companies and not by railways.

The crude shipment came from western Canada, and some of it was destined for Irving Oil's Saint John refinery, CN and Irving Oil have said.

Hallman said CN and the Transportation Safety Board of Canada were still investigating the nature of the damage to the tank cars and the volume of product affected.

While no one was hurt in Tuesday's accident, the derailment revived memories of a devastating crash last July, when a runaway train carrying light crude from North Dakota's Bakken region exploded in the heart of the town of Lac-Megantic, Quebec, killing 47. That train included DOT-111 cars.

The AAR has urged regulators to improve safety standards for tank cars carrying flammable liquids and has said old tank cars must be phased out, and that even new cars require modifications.

By contrast, energy producers have urged regulators to put more emphasis on track safety.

One leading trade group estimates that about 80,000 tank cars would need costly retrofits to comply with stricter standards being considered in the United States.

Cracked wheel

Guy Laporte, the investigator leading the TSB probe into the crash, said it was too early to say definitively what caused the derailment of the train, which was also carrying liquid petroleum gas. Four of the derailed tankers were carrying LPG.

CN said preliminary information indicated that a "wheel failure" was the cause. Laporte said that a broken rail had been found at the site of the accident.

"Yesterday we found a cracked wheel on a car near the front of the train," Laporte said. "This wheel moved on the axle and lost the track gauge resulting in the derailment of that wheel set. We also found a broken rail."

He added that only the front part of the train had been inspected so far.

CN said it was able to gain access to the site late on Wednesday and was able to move some of the cars that were not on fire overnight.

Two cars carrying LPG and one carrying crude were still burning on Thursday. There was also a smaller fire of locomotive fuel, CN said.